The present invention relates to a hydrocracking process for converting feedstock to lube oil base stock. More particularly, the present invention relates to hydrocracking using two different types of catalysts to produce a lube oil stock from a high boiling hydrocarbon feedstock at a lower cost than from either catalyst separately.
There are several properties of interest in a hydrocarbon oil to be of use for lube oil base stock. One is viscosity, which is a measure of how readily a fluid flows at a given temperature. Another, an empirically defined value, is the viscosity index, as defined by ASTM D-2270, hereinafter "VI," which measures how an oil's viscosity changes with changes in temperature. Pure samples, or mixtures such as petroleum oils, may have the same viscosity at a first temperature, but quite different viscosities at a second temperature. It is desired that lube oil base stock have as little change in viscosity with changing temperature as possible, and this is represented by a high VI, usually 90 or above. Less desirable oils may have large changes in viscosity with temperature, and, therefore have lower VI's. Originally the VI scale ran from 0 to 100, but oils of greater than 100 VI are known, as are oils of less than 0 VI.
It is known that paraffinic compounds have higher viscosity indices than do naphthenic or aromatic compounds. Crude oils, however, generally contain aromatic, naphthenic, sulfur, oxygen and nitrogen compounds as well as paraffinic ones. If a lube oil base stock with a high viscosity index is desired, it is necessary to selectively remove a considerable portion of these low VI components from the feedstock.
The use of hydrocracking to produce lube oil base stock is a standard process. The hydrocracking catalyst selectively cracks low VI components to products with boiling points below those of the feedstock. The high VI components are not cracked and, thus, are concentrated in the heavy product having a similar boiling range as the feed, but with improved qualities for lube oil base stock. The light cracked products can be separated from the heavy lube oil product by distillation.
Due to the increasing shortage of light crudes, feedstocks of low API gravity are now being utilized to make petroleum products. The 700.degree. F. to 1000.degree. F. boiling range portion of these feedstocks, typical for distillate lube products, tends to have less high VI components than feedstocks of the same boiling range heretofore used. This means more low VI compounds will need to be cracked out of the feed boiling range than previously to produce a high VI lube oil base stock. Hydrocracking conversion levels must be quite high. This requires very low space velocities when using conventional amorphous hydrocracking catalysts, which requires large reactors.
Other catalysts that can be used to produce lube oil base stock from hydrocarbon feedstock include catalysts which contain a crystalline zeolitic component. Typical crystalline zeolitic components include faujasite and mordenite dispersed in an amorphous cracking catalyst base. These catalysts are more active for cracking than the amorphous silica-alumina or alumina based catalysts and a larger amount of feedstock can be cracked with a given catalyst volume. One drawback with zeolite-containing catalysts, however, is that the zeolite-containing catalysts are not as selective as the catalysts that do not contain zeolite; that is, they tend to crack desirable high VI components as well as the undesirable lower VI components.
Other two-stage hydrocracking processes are known for producing petroleum products. U.S. Pat. No. 3,617,487 discloses a process for producing jet fuel by contacting a heavy feedstock with a zeolitic catalyst and then an amorphous catalyst.